A typical diketopyrrolo-pyrrolic pigment is C. I. Pigment Red described with a chemical formula as follows.

Among red color obtained by C. I. Pigment Red 254, pigments of various hues are known as an indicator with hue angle indicated by yellowish tint and blueish tint. In recent years, C. I. Pigment Red 254 which has stronger blueish tint and higher chroma is desired in the field of color coatings, plastics, and the like. However, the conventional C. I. Pigment Red 254 was insufficient.
Also, it is possible to obtain similar hue to C. I. Pigment Red 254 by toning organic pigments other than C. I. Pigment Red 254; and the chroma of the colored object obtained by toning became low, and no C. I. Pigment Red 254 having excellent chroma under intended hue was obtained.
Formerly, this type of C. I. Pigment Red 254 was produced by heating 1,4-diketo-3,6-bis(4-chlorophenyl)pyrrolo[3,4-c]pyrrole in organic solvent. The 1,4-diketo-3,6-bis(4-chlorophenyl)pyrrolo[3,4-c]pyrrole is a crude pigment obtained synthetically. As an organic solvent in this case, aprotic polar organic solvents of dimethylformamide or dimethylsulfoxide, for example, are known (as refer to Patent Document No. 1).
Also, a production process of C. I. Pigment Red 254 is known, where after crude pigment is added to mixture of methanol and sulfuric acid and is heated; then, this mixture is additionally heated in dimethylacetamide (as refer to Patent Document No. 2).
Also, a production process of opaque C. I. Pigment Red 254 is known, where crude pigment is wet ground by use of grinding medium at 50° C. or below in the presence of base in alcohol (as refer to Patent Document No. 3).
These production processes show either miniaturization in order to be effective particle diameter by grinding crude pigment of extremely large particle diameter, or contrary, regulation in order to be effective particle diameter as colorant by growing crystal of crude pigment with small particle diameter.
However, these production processes do not show intrinsic efficiency of the pigment, and it is hard to say that these production methods of C. I. Pigment Red 254 have excellent quality.
The chroma of C. I. Pigment Red 254 obtained by these production processes was still insufficient.
Cited DocumentsPatent DocumentJapanese Unexaminded Patent PublicationNo. 1;Application No. H04-372632 (page 2 claim, andparagraph [0010])Patent DocumentWO 03/022847 A2 (page 27 claim, and page 13No. 2;line 15-25)Patent DocumentJapanese Unexaminded Patent PublicationNo. 3;Application No. H01-306474 (page 1 claim, andfrom page 2 lower right column lowest line topage 3 upper left column line 12)